An active invisibility cloak — that is, one designed to work with an external power source — could significantly broaden the device's operation bandwidth, moving its applications beyond camouflage. A team at the University of Texas at Austin led by Andrea Alù, associate professor at the Cockrell School of Engineering, proposed a design for an active cloak that draws energy from a battery, allowing objects to become undetectable to radio sensors over a greater range of frequencies. That would give the proposed active cloak a number of new applications, such as improving cellular and radio communications, and biomedical sensing. Courtesy of The Cockrell School of Engineering at The University of Texas at Austin/Andrea Alù. Cloaks realized to date are passive, not designed to draw energy from an external source. They typically are based on metamaterials or metasurfaces (flexible, ultrathin metamaterials) that can suppress the light scattering off an object, making the object less visible. When the scattered fields from the cloak and the object interfere, they cancel each other out, and the overall effect is transparency to radio-wave detectors. They can suppress 100 times or more the detectability at specific design frequencies. Although the proposed design works for radio waves, active cloaks could one day be designed to make detection by the human eye more difficult, the UT team said. “Many cloaking designs are good at suppressing the visibility under certain conditions, but they are inherently limited to work for specific colors of light or specific frequencies of operation," said Alù, the David & Doris Lybarger Endowed Faculty Fellow in the Department of Electrical and Computer Engineering. But in the investigators’ work, “we prove that cloaks can become broadband, pushing this technology far beyond current limits of passive cloaks. I believe that our design helps us understand the fundamental challenges of suppressing the scattering of various objects at multiple wavelengths and shows a realistic path to overcome them.” Near-field setup of the cloaking device. Courtesy of The Cockrell School of Engineering at The University of Texas at Austin/Andrea Alù. The proposed active cloak uses a battery, circuits and amplifiers to boost signals, which makes it possible to reduce scattering over a greater range of frequencies. This design, which covers a very broad range, will provide the most broadband and robust performance of a cloak to date. Additionally, the proposed active technology can be thinner and less conspicuous than conventional cloaks, they said. In a paper published in Physical Review X in October, Alù and graduate student Francesco Monticone proved that existing passive cloaking solutions are fundamentally limited in the bandwidth of operation and cannot provide broadband cloaking. When viewed at certain frequencies, passively cloaked objects may become transparent, but if illuminated with white light, they are bound to become more visible with the cloak than without. The October paper proves that all available cloaking techniques based on passive cloaks are constrained by Foster's theorem, which limits their overall ability to cancel the scattering across a broad frequency spectrum. In contrast, the team said, an active cloak based on active metasurfaces, such as the one it designed, can break these limitations. The team started with a passive metasurface made from an array of metal square patches and loaded it with properly positioned operational amplifiers that use the energy drawn from a battery to broaden the bandwidth. “In our case, by introducing these suitable amplifiers along the cloaking surface, we can break the fundamental limits of passive cloaks and realize a ‘non-Foster’ surface reactance that decreases, rather than increases, with frequency, significantly broadening the bandwidth of operation," Alù said. The researchers are continuing to work on both the theory and the design behind their non-Foster active cloak, and they plan to build a prototype. They are working to use active cloaks to improve wireless communications by suppressing the disturbance that neighboring antennas produce on transmitting and receiving antennas. They have also proposed using these cloaks to improve biomedical sensing, near-field imaging and energy-harvesting devices. The team's most recent paper, "Broadening the Cloaking Bandwidth with Non-Foster Metasurfaces," was published Dec. 3 in Physical Review Letters. Alù, researcher Pai-Yen Chen and postdoctoral research fellow Christos Argyropoulos co-authored the paper. For more information, visit: www.utexas.edu